We have published a preliminary model of the HIV TAR RNA in complex with argininamide. However, several features of this RNA are poorly resolved. The wild-type TAR RNA has a three nucleotide bulge (UCU) that is critical for formation of the correct structure such that the HIV tat protein can bind. A naturally occurring TAR variant has a two nucleotide (UU) bulge, and we have prepared this RNA for structural characterization. It binds to argininamide somewhat more tightly than the UCU bulge TAR, and we observe several new inter- and intramolecular NOEs in the bulge region. After collecting a huge number of 2D-, 3D-, and 4D- data sets on "C-labeled RNA, and 15Nlabeled RNA, we have obtained a large number of distance constraints. There are over 700 NOEs that define the structure of this 30 nucleotide RNA, including 20 intermolecular NOEs to the argi ninamide ligand. We have completed the structure determination, and the paper describing the structure is in press. The first U nucleotide in the bulge region forms a base triple with an A-U base pair in the upper stem, and the arginine binds immediately below this U base, interacting with a G-residue in the major groove. The guanidinium. group is stacked between two bases, forming an arginine sandwich.